Meta

Archive for June, 2014

Bateman’s principles are conceptually quite simple, but form the basis of our understanding of sexual selection across the animal kingdom. First proposed in 1948, Bateman’s three principles posit that sexual selection is more intense in males than in females for three reasons:

1) males show more variability in the number of mates they have (mating success);
2) males show more variability in the number of offspring they have (reproductive success);
3) the slope of the relationship between mating and reproductive success is steeper in males;

Together, this summarises our basic view of sexual selection in the majority of sexually reproducing species – males that do well, do very well and we expect more intense sexual selection because of it.

Biased Traditions
Traditionally, most studies investigating these relationships have measured mating success by counting the number of females a male produces offspring with. This method is biased though, as it assumes that every mating results in offspring, which is unlikely to be true. Further, it assumes that every fertilisation produces an offspring, which ignores cases where embryos die before birth. Using offspring counts as a way to measure mating success might not be accurate but it is certainly more practical – behavioural observations of actual mating would be very time consuming and nearly impossible for some species. However, until now no study has attempted to quantify the importance of these biases in calculating and testing Bateman’s principles.

Carefully Observed
To address this issue, GEE researchers Dr Julie Collet and Dr Rebecca Dean, in collaboration with researchers at the University of Oxford, University of Queensland, Uppsala University and the University of East Anglia, investigated mating and reproductive success in Red Junglefowl (Gallus gallus). They recorded matings and collected all eggs laid from 13 groupings of 3 males and 4 females (mimicking natural conditions). They began by using classic techniques to estimate Bateman’s gradients – they inferred mating success from the number of females they sired an offspring with. They found twice as much variability in male mating success, and four times as much variance in male reproductive success (the actual number of offspring a male produced) compared with females. Mating success and reproductive success were strongly related – differences between individuals in mating success explained 57% of variance in reproductive success in males, but only 24% in females, and the slope of the relationship was steeper in males.

A Male Red Jungle Fowl

They then repeated their analysis but with a more accurate measure of mating success – the actual number of partners and matings observed. In this study, 30% of pairs that mated did not produce any offspring together and would be ignored by the traditional measure of ‘mating success’. Including these matings reduced the variability in mating success in both males and females. It also reduced the explanatory power of mating success – using this technique they found that variation in mating success actually explained 43% of variation in male reproductive success, and just 5% of variation in female reproductive success. This suggests that traditional methods for measuring Bateman’s principles are likely to be overestimating their importance and the extent of sexual selection on males.

Covarying Factors
Reproductive success is not just a product of how many mates you have. The fecundity of your mate is also a crucial factor, and in species where females mate with multiple males, your share of her offspring is also a key variable. The authors investigated whether these variables tend to be related and whether multivariate analyses that take them all into account better explain the overall reproductive success of a male. Their multivariate model explained the variance in male mating success better than the standard approach and found that mating success, paternity share and mate fecundity together are responsible for the variance in male reproductive success. The authors estimate that by ignoring these other factors, other studies may overestimate the Bateman gradient by as much as 150%!

This study shows the importance of investigating the biases we introduce into our science. These biases may sometimes be inevitable, if excluding them is extremely time consuming or difficult. But we must try to understand the influence of these biases in order to draw informed conclusions from our data. Here, GEE researchers demonstrate how using biased measures of mating success can cause scientists to overestimate the opportunity for sexual selection on males. This effect is likely to be largest for species which have small clutch sizes and in which sperm competition plays a key role. Where possible, studies investigating sexual selection should include accurate measures of mating success, and include other variables such as paternity share and mate fecundity in a multivariate approach in order to best understand Bateman’s principles and the relationship between mating and reproductive success in both sexes.

Many of our greatest technological advances have tended to mark disaster for nature. Cars guzzle fossil fuels and contribute to global warming; industrialised farming practices cause habitat loss and pollution; computers and mobile phones require harmful mining procedures to harvest rare metals. But increasingly, ecologists and conservation biologists are asking whether we can use technology to help nature. On 10th June 2014, UCL’s Center for Biodiversity and Environment Research (CBER) hosted academics from the National Museum of Natural History, Paris, the Zoological Society of London and the Natural History Museum, London for a workshop on “Technology for Nature”. The workshop formed part of a series of public debates and workshops organised in collaboration with the French Embassy, around the theme of the ‘State of Nature’.

The workshop discussed some of the latest technologies available to monitor biodiversity and how these might be harnessed in combination with citizen science to better understand the natural world around us. Citizen science, which engages members of the public in collecting and processing data about nature, is a powerful tool enabling scientists to collect much larger quantities of data on populations of key species. Citizen science projects not only provide biologists with vastly more data than they could ever hope to collect on their own, but it also serves to engage members of the public with the natural world, and raise awareness of key environmental issues.

Following the workshop, UCL also hosted an evening debate in collaboration with colleagues at the French Embassy, London. Professor Romain Julliard from the National Museum of Natural History, Paris, and Professor Kate Jones from UCL’s CBER discussed how new technologies can be used to understand and predict the impact of humans on the natural world, and whether these technologies can be used to inspire and engage the public with the environment around them.

Professor Julliard is the Scientific Director of Vigie Nature, a project to monitor trends in various widespread species including butterflies, birds, bees and flowers, using citizen scientists in France. Professor Jones holds the chair of Ecology and Biodiversity at UCL and the Zoological Society of London, and has been involved in a number of projects utilising citizen scientists to monitor populations of bats both in the UK and across Europe. She started the iBats project, which uses volunteers to collect acoustic recordings of bat calls which a computer algorithm can then use to identify the species, and has used citizen science to process data from this project through Bat Detective.

The meeting last week brought together academics from a range of different institutions with a shared interest in monitoring biodiversity to better understand how humans are impacting upon it. We hope this will lead to new projects and collaborations to monitor biodiversity and gain vital data that is needed to assess and ultimately mitigate our impact on the animals and plants we share the planet with.

While it might seem as though our genes are all working together for our own good, some of them are actually rather selfish. Scientists have known about ‘selfish genetic elements’ for nearly a century, but research to understand their behaviour and effects is ongoing. Recent research in GEE reveals how sexually selected traits are signalling selfish genetic elements (or a lack of them) in the same way they are used to signal male quality and health.

Selfish Genes and the Balance of the Sexes
Selfish genetic elements are variants (alleles) of genes which, rather than acting to the benefit of the individual, act in their own interest to ensure maximum replication of themselves. One type of selfishness that genes can exhibit is called meiotic drive, and is associated with alterations to the sex ratio of offspring. Sex ratios for most species tend towards 1:1, for sound evolutionary reasons, but this isn’t in the best interest of all genes. Genes that lie on one of the sex chromosomes (X or Y in mammals, Z or W in birds) are not equally successful in both sexes – if you happen to lie on the X chromosome, for example, there’ll be two copies of you in each female offspring but only a single copy in male offspring. Meiotic drive occurs when selfish genetic elements skew the sex ratio in order to favour their own replication – usually caused by genes on the X chromosome creating a female-biased sex ratio.

Sex ratios tend to be roughly even in nature because offspring sex ratio is a trait that is under strong stabilising selection. In populations with a biased sex ratio, the underrepresented sex immediately becomes extremely valuable, simply by virtue of its rarity. Imagine a village consisting of 20 women and a single man – that man would undoubtedly have the pick of the ladies, and would likely produce more offspring. If you are the mother of that male, you’ll have done very well for yourself. In a population with a skewed sex ratio, offspring of the underrepresented sex are more valuable, and natural selection to produce them is very powerful. Any mechanism by which females could identify a male that will produce these valuable offspring would be strongly favoured by selection.

A Male Stalk-Eyed Fly (Teleopsis dalmanni)

Meiotic Drive
Stalk-eyed flies are found in Africa and Southeast Asia and show striking sexual dimorphism (physical differences between the sexes). Both males and females have their eyes placed on the end of long stalk-like appendages, but in males these ‘stalks’ can be very long. This is a sexually selected trait – males with longer eyestalks are healthier, carry fewer harmful mutations and are better at attracting females, meaning they tend to have more offspring.

In the laboratory, stalk-eyed flies often produce very female-biased broods; this is thought to be a result of meiotic drive that causes male sperm (carrying a Y chromosome) to degenerate, but female sperm (carrying an X chromosome) to persist. Researchers believe that the length of eyestalks may be linked to meiotic drive, and long-eye stalks may signal to females that a male does not carry the harmful selfish allele. In the laboratory, it has already been shown that males selectively bred for short-eye spans tend to produce female-biased broods, and four loci on the X-chromosome have been identified that are associated with female-bias.

Beyond the Laboratory – Tests in Wild Populations
Expanding on this research, academics in GEE wanted to investigate this phenomenon in the wild. Their work, recently published in Heredity, investigated eyespan and sex ratio biases in 12 populations of wild stalk-eyed flies in Malaysia. Dr Alison Cotton and colleagues at UCL and the University of Debrecen, Hungary, collected nearly 500 wild stalk-eyed flies, measured their eyestalks and other physical characteristics, and collected DNA samples. The researchers used a technique known as microsatellite genotyping to identify regions of the X-chromosome where genes responsible for meiotic drive and for eyestalk length were located. Microsatellites are repeating DNA sequences that tend to vary in their length (the number of repeats) between individuals. Microsatellites can be used as genomic markers for nearby genes of interest – we expect that different microsatellite alleles will be consistently associated with different alleles of interesting genes nearby. Because they vary in physical length, microsatellite alleles can easily be identified by separating DNA sequences out according to size.

The authors found that one microsatellite, ms395, was strongly associated with male eyestalk length. This relationship was not found in females. Longer ms395 alleles tended to be associated with smaller male eyespans.

Stalk-eyed flies, (Teleopsis dalmanni)

Males collected from 5 wild populations were taken back to the lab where they were allowed to mate, so that researchers could investigate the sex ratio of their offspring. Around a quarter of wild-caught males produced biased sex ratios, most often producing more females than males. Males producing sex-biased offspring tended to have smaller eyestalks, when their overall body size was controlled for. They also tended to have longer ms395 alleles.

The authors were able to show that microsatellite ms395 is associated both with sex ratio biases and with male eyestalk length, suggesting that the genes controlling eyestalk length and meiotic drive are located on the X-chromosome near ms395, and that eyespan may be a signal of the genetic quality of the male. For females, a male that carries a selfish genetic element that causes meiotic drive (and a lack of male offspring) is of poor genetic quality, but by preferentially mating with males with longer eyestalks, females can avoid these harmful genes. This may indicate that eyestalks are an example of the good genes hypothesis for sexual selection, which suggests that physical characteristics involved in mate choice are associated with alleles that produce high quality males and are therefore used as an honest signal of the quality of potential mates.

This study is the first to demonstrate meiotic drive in wild populations of the stalk-eyed fly (Teleopsis dalmanni) and adds strong support to previous research suggesting that male eyestalk length is a signal of the presence of meiotic drive. As long as these genes remain in close proximity to each other through evolutionary time, the association should be maintained and eyestalks can be used as a reliable way for females to identify good quality males. Or at least males that are likely to give them a nice even sex ratio in their offspring.